Custom coolwsd.xml for built-in Nextcloud Office

I want to install a font on my Nextcloud for use with Nextcloud Office. In Administration > Nextcloud Office > Extra Fonts I’ve uploaded the fonts. Below those fonts it says:

Make sure to set this URL: https://<MY_URL>/apps/richdocuments/settings/fonts.json in the coolwsd.xml file of your Collabora Online server to ensure the added fonts get loaded automatically.

I run Nextcloud via Docker. Directly editing the file in the container doesn’t seem to persist after a restart. I tried to mount coolwsd.xml, but Collabora then doesn’t start (Could not establish connection to the Collabora Online server. is shown in the Nextcloud Office settings).

Is it possible to use my own fonts with my setup? Another question, why isn’t coolwsd.xml automatically configured with the URL? That would make sense for the built-in version right?

EDIT: no response here, so I opened a feature request: Custom fonts for built-in Nextcloud Office · Issue #2763 · nextcloud/richdocuments · GitHub